1. Field of the Invention
The invention relates to a light emitting diode (LED), and more particularly to an LED that emits light of so-called electric bulb color such as white, off-white, light blue, light yellow, etc.
2. Description of the Related Art
In recent years, there is an increasingly strong demand for power-saving and long-lasting lighting equipment from the viewpoint of preventing global warming, effectively using resources, and so on. In response thereto, LEDs are rapidly becoming shorter in wavelength and higher in brightness. Particular hope is placed on white LED's that use blue LED's for finding application in lighting.
White LEDs that have a shell-shaped or surface-mount configuration are conventionally known. The conventional white LED is designed to externally emit white light by converting light from a blue LED chip to yellow light with a phosphor layer, and mixing the yellow light with blue light from the blue LED chip to create the white light.
A conventional shell-shaped white LED is configured, for example, as shown in FIG. 7. That is, in FIG. 7, a white LED 1 includes a pair of lead frames 2 and 3, and a blue LED chip 4 mounted on top of a chip mounting portion 2a formed on the upper end surface of the lead frame 2. A phosphor layer 5 is formed surrounding the blue LED chip 4 on top of the chip mounting portion 2a of the lead frame 2 and includes phosphor 5a mixed therein. A lens portion 6 is formed with mold resin so as to surround the upper ends of the lead frames 2 and 3, the blue LED chip 4, and the phosphor layer 5.
The lead frames 2 and 3 are formed with a conductive material such as aluminum and are provided with the chip mounting portion 2a and bonding portions 2b and 3a at respective ends thereof. The other ends of the lead frames extend downward to make up terminal portions 2c and 3b. 
The blue LED chip 4 is joined on top of the chip mounting portion 2a of the lead frame 2, with two electrodes provided on the upper surface thereof electrically connected to the bonding portions 2b and 3a at the ends of the lead frames 2 and 3 by bonding wires 4a and 4b. Here, the blue LED chip 4 is configured, for example, as a GaN chip and designed, when applied with a drive voltage via the lead frames 2 and 3, to emit light having a peak wavelength of about 450 to 470 nm.
The phosphor layer 5 is made, for example, of clear epoxy resin into which the phosphor 5a in fine particulate form is mixed. The phosphor layer 5 is formed and hardened on top of the chip mounting portion 2a of the lead frame 2.
As blue light from the blue LED chip 4 falls on the phosphor layer 5, the phosphor 5a is excited, producing yellow light from the phosphor 5a and externally emitting white light as a result of mixing of the two lights. Here, the phosphor 5a includes a phosphor that emits a wide range of lights centering around yellow light such as YAG phosphor doped with cerium, TAG phosphor doped with cerium or orthosilicate phosphor (BaSrCa) SiO4, and is designed to produce a fluorescence, for example, with a peak wavelength of about 530 to 590 nm.
The lens portion 6 is made, for example, of clear epoxy resin, and is formed such that it surrounds the whole area near the upper ends of the lead frames 2 and 3 centering around the blue LED chip 4 and the phosphor layer 5.
Based on the white LED 1 thus configured, the blue LED chip 4 emits light when a drive voltage is applied via the pair of lead frames 2 and 3. The light falls on the phosphor 5a mixed into the phosphor layer 5, exciting the phosphor 5a and producing yellow light. Then, this yellow light is mixed with blue light from the blue LED chip 4, thus causing the mixture to be externally emitted as white light. In this case, white light has a spectrum distribution, for example, as shown in FIG. 8.
On the other hand, a surface-mount white LED 7 can be configured, for example, as shown in FIG. 9. In FIG. 9, the white LED 7 includes a chip substrate 8, a blue LED chip 4 mounted on top of the chip substrate, a frame-shaped member 9 formed on top of the chip substrate 8 so as to surround the blue LED chip 4, and a phosphor layer 5 charged into a depressed portion 9a of the frame-shaped member 9.
The chip substrate 8 is made of a heat-resistant resin as a flat copper clad wired board, and is provided with a chip mounting land 8a and an electrode land 8b on the surface. Surface-mount terminal portions 8c and 8d extend around from these lands onto the lower surface via both end edges. The blue LED chip 4 is joined on top of the chip mounting land 8a of the chip substrate 8, with the blue LED chip 4 electrically connected to the chip mounting land 8a and the electrode land 8b through wire-bonding.
The frame-shaped member 9, similarly formed on top of the chip substrate 8 with a heat-resistant resin, is provided with a recessed portion 9a—a portion in the form of an inverted truncated cone—so as to surround the blue LED chip 4. It is to be noted that the inner surface of the recessed portion 9a is configured as a reflecting surface.
Based on the white LED 7 thus configured, the blue LED chip 4 emits light when a drive voltage is applied via the surface-mount terminal portions 8c and 8d, causing light to fall on the phosphor 5a mixed into the phosphor layer 5, exciting the phosphor 5a and producing yellow light. Then, this yellow light is mixed with blue light from the blue LED chip 4, thus causing the mixture to be externally emitted as white light.
However, there are problems with the white LEDs 1 and 7 configured as described above. For example, blue light emitted from the blue LED chip 4 is converted in wavelength by the phosphor 5a to produce yellow light, with blue and yellow lights mixed together to emit white light. This white light has a color temperature, for example, of 5000 to 6000K. In contrast, an incandescent lamp (a lamp conventionally used over the last 100 plus years), has a color temperature, for example, of 2800 to 3000K.
Incidentally, when a conventional white LED lamp is used in place of an incandescent lamp in lighting equipment, the white LED produced light appears as a bluish white light unlike the so-called electric bulb light color for an incandescent lamp (which appears as a warm-looking color tinged with red). This is true because the conventional white LED light has insufficient light intensity in the red region, as shown in FIG. 8, due to relatively high color temperature as described above, thus giving a cold impression.
On the other hand, while a red phosphor that produces red light by excitation with blue light—a recent development—may be used, red phosphors are generally made of alkaline earth metal and therefore vulnerable to humidity, making it difficult to configure a highly reliable LED and difficult to obtain a sufficient intensity of red light.